1. The Field of the Invention
The present invention relates to soft tissue repair surgery, such as rotator cuff repair surgery. More specifically, the present invention relates to bone anchors for attaching soft tissue to bone using a suture.
2. Related Technology
Soft tissue injuries, especially rotator cuff injuries, can occur from repeated stress or acute trauma. The rotator cuff is a group of muscles and tendons in the shoulder that attach to the humerus bone of the arm. The rotator cuff allows a person to rotate the arm and raise it above the head. A common injury to the rotator cuff occurs when repeated stress or acute trauma causes the rotator cuff to partially or complete tear away from the humerus bone. These and similar types of injuries may require surgery to correctly reattach the soft tissue to the bone.
Various devices have been used to reattach soft tissue to bone. Known methods include staples, wedges, inserts, screws, and sutures alone. Threaded suture anchors, such as bone screws, have recently been developed to provide a particularly firm location where a suture can be anchored to bone. In these systems, a suture is tied between the bone anchor and soft tissue. Providing a firm attachment point for the suture is important because of the relatively strong forces that are experienced in a flexing muscle.
Despite recent advances in bone anchors, existing bone anchors and rotator cuff repairs can fail and have other disadvantages. Typically, a rotator cuff repair fails either because the bone anchor dislodges or the suture tears through the soft tissue. As force is applied to the suture, the suture can cut through the soft tissue like a cheese wire, leaving the rotator cuff detached from the humerus bone. When one suture fails, it can place more stress on the surrounding sutures, thus increasing the likelihood that other sutures will fail in like manner.
Using a greater number of sutures per unit area of soft tissue can minimize suture attachment failure. However, the number of sutures that can be used in a particular procedure is limited by the number of bone anchors that can be used and the number of sutures per anchor. During a surgical procedure, the bone anchors are placed in the humerus bone where the tissue has torn away. Thus, the number of bone anchors is limited to the number of anchors that will fit in the repair area without compromising the integrity of the humerus bone. In most rotator cuff repair surgeries, this number is two or three anchors.
The number of sutures that can be attached to each bone anchor depends on the configuration of the bone anchor. Double loading of bone anchors has been achieved for some bone anchors and is currently preferred over single loading. Typically these bone anchors have an eyelet or loop and the sutures are attached by threading the sutures through the eyelet or loop.
Loading multiple sutures on an eyelet or loop is problematic because it can cause “suture capture.” During a rotator cuff repair procedure, a practitioner typically ties a knot that can be adjusted by sliding. To properly tie and adjust the knot, the suture must be able to slide in the bone anchor. Multiple sutures loaded on an eyelet or loop create friction and/or pinching that can prevent the suture from sliding in the bone anchor (i.e. causes “suture capture”). Suture capture occurs most often at the point where the sutures are sliding on the eyelet or loop of the bone anchor and after the first suture has been tied. If a first suture is tied with it crossing over an untied suture the first suture can cause friction and/or pinch the second suture against the eyelet or loop, thereby causing suture capture. As the number of sutures increases the likelihood of suture capture also increases.
Suture capture can be particularly disruptive to a surgical procedure. In many cases, if a suture becomes captured the suture can break when the practitioner attempts to adjust the knot. Because the bone anchors are small and the bone anchor is in the bone of a human when the suture breaks, it is not possible for the practitioner to thread the bone anchor with a new suture. Furthermore, it is very undesirable to remove the bone anchor once it is in place, especially if one of the sutures in the anchor has already been tied. In such cases, the bone anchor is typically left in place with only one suture. The reduced number of sutures places more strain on the soft tissue, which can increase the risk of tissue failure.
Recently, bone anchors that are less likely to cause suture capture have been developed. These anchors have two separate attachment sites inside a longitudinal bore. Attaching sutures within a longitudinal bore is particularly advantageous because it allows the bone anchor to be fully threaded (fully threaded bone anchors have threads near the proximal end that can be embedded in hard cortical bone). However, attaching sutures at different locations inside a longitudinal bore is very difficult due to size restraints. The outer diameter of a bone anchor is typically between 5.0 mm and 7.0 mm. This maximum outer diameter limits the bore diameter to between about 2.5 and 4.0 mm. Due to these size restraints, exiting bone anchors have been limited to a maximum load of two sutures.